Recent progress in the field of GPS has enabled GPS receivers to be produced relatively cheaply leading to their widespread adoption and use. For example, one might envisage a home networking environment in which is provided a mobile telephone with GPS capability for providing its location to a cellular telephone network operator in-the event of an emergency call; a TV with GPS capability for providing TV access control, say as described in U.S. Pat. No. 5,621,793; and a personal computer with GPS and Internet capability for retrieving location specific information from a web site, say a local weather report.
In such a home networking environment, three GPS receivers are provided, each returning substantially the same location. However, especially indoors, it is unlikely that all three GPS receivers will each be able to acquire the four GPS space vehicle (SV) signals normally required to obtain a position fix, or at least not all of the time. One reason for this is obscuration of the GPS signals by buildings, walls and other urban paraphernalia.
U.S. Pat. No. 5,451,964 (“Babu”) discloses a mobile GPS receiver station operating with a reference GPS receiver station wherein reference station pseudoranges and co-ordinates are transmitted from the reference station to the mobile station. In particular, Babu discloses where the reference pseudoranges are used to generate pseudorange corrections at the mobile station which are then applied to corresponding mobile station pseudoranges, themselves used to determine the position of the mobile station. By corresponding, one means that Babu discloses uses reference station pseudoranges to make corrects to mobile station pseudoranges necessarily obtained from the same satellites. Babu is in effect an implementation of differential GPS (DGPS) but slightly differently from conventional DGPS implementations in that the differential corrections are applied to individual mobile station pseudoranges rather than the resultant position fix.
It is an object of the present invention to provide a method of determining the location of a device which is more effective indoors.